1. Field of the Invention
The present invention relates to a local oscillation circuit, without a phase locked loop (PLL) circuit, for providing an oscillating signal having a stable amplitude, and also relates to a receiving circuit, for mobile communication, including the local oscillation circuit.
2. Description of the Related Art
Receiving circuits include local oscillation circuits. In the prior art, local oscillation circuits include PLL circuits and oscillation circuits driven by the outputs of the PLL circuits. A PLL circuit and circuits other than an oscillator circuit may be integrated into one semiconductor chip. However, an oscillator circuit cannot be integrated into the semiconductor chip because an oscillator circuit includes many external parts such as variable capacitors, coils, capacitors, etc. Therefore, the prior-art local oscillation circuit has a problem in that it has a large number of circuit parts; the receiving circuit can not be made into one semiconductor chip; the size of the local oscillation circuit is large and, accordingly, the size of the receiving circuit is large.
In view of the above-mentioned problems in the prior art, an object of the present invention is to provide a receiving circuit which can be made as a single semiconductor chip by employing a voltage-current converting interface part in a local oscillation circuit in the receiving circuit so as to reduce the number of parts in the local oscillation circuit part.
Another object of the present invention is to provide a receiving circuit which consumes little current.
Still another object of the present invention is to provide a receiving circuit in which the sensitivity with respect to an input signal from an antenna is independent of the power supply voltage.
To attain the above objects, there is provided, according to the present invention, a local oscillation circuit comprising a crystal oscillating circuit-for generating an output voltage having a constant frequency, and an interface part for converting the output voltage from the crystal oscillating circuit into a current signal, the current signal being used as a local oscillation signal to be mixed with the receiving signal from an antenna.
Since no PLL circuit is included in the local oscillation circuit, the number of parts in the local oscillation circuit can be made small in comparison with the local oscillation circuit including the PLL circuit. Further, since the voltage is converted into a current, the fluctuation of the amplitude of the oscillating voltage can be limited even when a low pass filter and a voltage controlled oscillator (VCO) are not employed. Therefore, the local oscillation circuit can be operated even when the power supply voltage is a low voltage, so that a local oscillation circuit consuming little current can be realized, resulting in a long life of a battery in the circuit or in a miniaturization of the battery.
Preferably, the interface part comprises a constant current source for converting the output voltage from the crystal oscillating circuit into a square wave signal having a frequency corresponding to the frequency of the output voltage, a filtering part for removing high frequency components in the square wave signal output from the constant current source, and a current interface part for converting a change in the voltage of the signal close to a sine wave output from the filtering part into a change in current.
By employing the constant current source which can provide a constant gain even when the power supply voltage is low, the fluctuation of the current due to variations of the manufacturing processes can be limited so that the local oscillation circuit can be incorporated into a receiving circuit to provide a stable input sensitivity independent of the power supply voltage.
Still preferably, the constant current source comprises a constant current source part connected to a power supply line, a load connected to the ground, and a switching part, connected between the constant current source and the load, which can be turned ON or OFF in response to the output voltage from the crystal oscillating circuit. By this construction, a square wave voltage having a desired amplitude can be obtained across the load.
Further preferably, the constant current source comprises a differential pair of transistors connected to the power supply line, and a power source for supplying a constant current to the differential pair of transistors. The constant current is independent of the temperature. The switching part is a switching transistor connected between one of the differential pair of transistors and the load. By this construction, in response to the voltage output, from the crystal oscillating circuit, to be input into the switching transistor, a current flows through one of the differential pair of transistors and the load.
Still further preferably, the current interface part comprises a first differential pair including a first transistor having an input to receive the output voltage from the filtering part and a-second transistor having an input to receive a reference voltage, a current supplying source for supplying a current to the first transistor and the second transistor in response to the output voltage from the filtering part, and a second differential pair including a pair of a third transistor and a fourth transistor for differentially passing a current from the current supplying source in response to the operation of the first differential pair. In this construction, a current flowing through the second pair is the local oscillation signal.
According to another aspect of the present invention, there is provided a receiving circuit comprising the above-mentioned local oscillation circuit, an antenna for receiving a signal, and a mixer circuit for mixing the output current from the interface part with a receiving signal from the antenna.
Preferably, the mixing circuit comprises a local oscillation interface circuit for conducting a current in response to an output current of the interface part, a mixing part for mixing a receiving signal from the antenna with a current flowing through the local oscillation interface circuit; and an output circuit for conducting a constant current through the mixing part.